The present invention relates to circuits for controlling the horizontal width of the raster of a CRT image display apparatus.
CRT image display apparatus such as those used in video monitors and television receivers typically include a set of deflection coils mounted on the neck of the tube which are energized to produce electromagnetic fields for horizontally and vertically deflecting the electron beam produced by the CRT electron gun across the CRT screen. In order to produce the desired scanning pattern, a current signal having a waveform comprising a repetitive sequence of sawtooth pulses is induced in the deflection coils, the amplitude and width of the sawtooth pulses determining the extent of deflection of the electron beam across the CRT screen. The sawtooth current pulses are normally induced in the deflection coils in response to signals developed at the output of a sweep amplifier which, in turn, is operated in response to a horizontal or vertical oscillator whose oscillatory frequency is synchronized to the processed video signal.
In such display apparatus, it frequently becomes necessary to provide a facility whereby the extent of deflection, particularly in the horizontal direction, can be manually set or adjusted. Prior art attempts to provide such an adjustment capability have generally taken one of two forms. In one technique, a variable, relatively low impedance inductor is connected in series with the horizontal deflection coil increasing the total impedance seen at the output of the horizontal sweep amplifier. Through a voltage divider action, the voltage developed across the horizontal deflection coil, together with the current flowing therethrough, is reduced thereby decreasing the amplitude of the sawtooth current pulses and the extent of horizontal deflection. Increasing the inductance of the variable inductor further decreases the amplitude of the sawtooth current pulses and the extent of horizontal deflection while decreasing the inductance increases the amplitude of the sawtooth current pulses increasing the extent of horizontal deflection.
In another prior art technique, a variable, relatively high impedance inductor is connected in shunt with the horizontal deflection coil dividing the sawtooth current pulses between the deflection coil and the shunting coil thereby reducing the sawtooth current in the deflection coil and the extent of horizontal deflection. Decreasing the inductance of the variable inductor further decreases the sawtooth current signal and the extent of horizontal deflection while increasing the inductance increases the sawtooth current signal increasing the extent of horizontal deflection.
In both cases, i.e. the prior art series width control and the prior art shunt width control, it will be appreciated that the variable control inductor loads the horizontal sweep amplifier and that the loading effect changes for different settings of the control inductor. In particular, in the prior art series width control circuits, the impedance seen at the output of the horizontal sweep amplifier is directly related to the setting of the variable control inductor while, in the prior art shunt width control circuits, the impedance is inversely related to the setting of the variable control inductor. Since the horizontal deflection circuits are designed for optimum operation when connected to an output impedance having a certain constant level, the effect of varying the inductance of the variable control inductor for adjusting the extent of horizontal deflection is to detune the horizontal deflection circuits and thereby degrade their operation. In addition, it has been found that the range of horizontal width control which may be realized using either prior art circuit is generally insufficient for many applications.
It is accordingly a primary object of the present invention to provide an improved circuit for controlling the extent of horizontal deflection in an image display apparatus. It is a more specific object of the invention to provide an improved circuit of the foregoing type whose operation does not significantly detune the horizontal deflection circuit of the image display apparatus and which provides an increased range of horizontal width control.